Heat exchangers are commonly used in gas fired hot air furnaces in both residential and commercial settings. Heat exchangers are generally divided into two types. The first includes tubular heat exchangers where a tube is formed in a serpentine configuration and hot combustion gases are allowed to propagate through the tube. The second type of heat exchanger includes a compact design which may have a clam shell construction.
In typical use in a furnace, a series of heat exchangers are provided in which hot combustion gases pass through the exchangers transferring heat to the surfaces of the heat exchanger. Forced air passed externally over the heated surfaces of heat exchangers is warmed and circulated into a room which is to be heated. The efficiency of the heat exchanger is dictated by the effectiveness of the transfer of heat from the hot combustion gases within a heat exchanger to the external surfaces of the heat exchanger itself.
Also, many furnaces employ secondary heat exchangers which are used to extract added heat from the combustion gas exiting the primary heat exchangers.
As may be appreciated, it is desirable to increase the heat transfer between the combustion gases and the walls of the primary and secondary heat exchangers.
One such example is shown in U.S. Pat. No. 6,938,688 which employs a clam shell design for primary heat exchangers where turbulent flow of the combustion gases is caused. This results in more efficient heat transfer.
However, as may be appreciated, such techniques may increase the size of the heat exchanger. Thus, additionally employing such a design for secondary heat exchangers would increase both the size and cost of the furnace.
It is, therefore, desirable to provide an increase in the heat transfer surface area of a heat exchanger that is exposed to the combustion gases without increasing the external size of the heat exchanger itself.